A. Field of the Invention
The present invention relates to fabrication of buckypaper or an analogous sheet or layer with some degree of alignment of nanostructures having some elongation in at least one direction and, in particular, to utilization of fluid flow dynamics to influence filtered collection of the nanostructures from a suspension of the nanostructures.
B. Related Art
Nanostructures are materials with at least one dimension on the order of nanometers in scale. Much work is being done to develop applications for them, either individually or in agglomeration. Nanostructures with elongation in one direction can be fibers (nanofibers) or other structures having an axis of elongation or an aspect ratio well above 1. One example is tubes (nanotubes). Another example is nanocellulose fibrils. Another example is nanoribbons.
Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. They are members of the fullerene structural family and resemble graphitic sheets rolled into tube shapes. Rolling angle and radius influence their properties. They have been found valuable for, inter alia, nanotechnology, electronics, optics and other fields of materials science and technology.
Both single-walled (SWCNT) and multi-walled (MWCNT) varieties of CNTs have been synthesized and both exhibit outstanding mechanical, thermal, and electrical properties. These extraordinary properties have prompted a great deal of research into the efficient synthesis of CNTs, and today high quality MWCNTs are commercially available at huge quantities and low cost (˜$120/kg). As a result, high-volume engineering applications of MWCNTs are becoming a reality after decades of promise at the laboratory scale. One of the most promising immediate applications of MWCNTs is as filler in composites, specifically polymer matrix composites. However, composites fabricated by mixing multi-walled carbon nanotubes (MWCNTs) into a resin are limited to low loading levels because the large increases in viscosity that occur at higher loadings encumber processing. This, in turn, limits the effect that MWCNTs can have on the composite properties, and new methods must be developed if the true potential of CNT composites is to be realized. One way to achieve high loadings of CNTs in a composite is through the use of buckypaper (BP), which is a free-standing mat of tightly packed CNTs formed by the controlled filtration of CNT solution. FIGS. 1A and 1B show the typical method used to produce BP and optical images of the BP itself. BP can be handled in a manner similar to glass and carbon fiber mats, and traditional composite processing techniques such as compression molding and vacuum-assisted resin transfer molding can be used to infiltrate resin into the pores of the BP mat and bind several plies together into composites. See references [3, 4] (bracketed numbers refer to the listing of References later in this description). See also L. Hussein et al., Phys. Chem. Chem. Phys. 13, 5831 (2011), incorporated by reference herein. Loadings up to 60 wt % MWCNTs have been achieved [5] and outstanding mechanical, [6,7] thermal, [8] electrical, [7] and electromagnetic shielding properties [9] have been realized in BP/epoxy composites. Most BP is composed of CNTs that are randomly aligned. However, as with any fiber-reinforced composite, optimal properties are realized when the fiber alignment is unidirectional within each ply and the composite layup is judiciously tailored to match the expected stress state of its application.
However, as well-recognized by those in this technical field, the extremely small size of nanostructures and their properties present substantial challenges regarding their handling. What might work with macro-sized discrete items may not work with nano-sized structures.
Three approaches currently exist for the production of aligned BP mats: alignment through mechanical stretching of cross-linked CNT mats, “domino pushing” of aligned CNT forests, and magnetic alignment. Mechanical stretching involves uniaxially straining randomly aligned MWCNT BP and then impregnating it with resin. Bismaleimide (BMI)/BP composites made with this process possessed outstanding mechanical and electrical properties, [5] and, when the MWCNTs in the BP were functionalized with epoxide groups, the resulting composites exhibited unprecedentedly high strength (3081 MPa) and modulus (350 GPa), surpassing even high-performance carbon fiber composites. [10] See illustration at FIGS. 2A and 2B and [34] (see D. Wang et al. Nanotechnology 19, 609 (2008), incorporated by reference herein). However, the MWCNTs used in this study were cross-linked together through a specialized synthesis process necessary to prevent the BP from tearing at high strains, which excludes the method from widespread industrial use in the near future. Additional discussion of mechanical stretching can be found at Cheng Q, Bao J, Park J, Liang Z, Zhang C, Wang B. High Mechanical Performance Composite Conductor: Multi-Walled Carbon Nanotube Sheet/Bismaleimide Nanocomposites. Advanced Functional Materials. 2009; 19(20):3219-25 [5] (use of mechanical stretching or drawing to align CNTs), which is incorporated by reference herein.
Highly aligned BP can also be produced through “domino pushing” of MWCNT forests. See illustrative at FIG. 3. In this method, vertically aligned MWCNT forests are grown on a Si substrate and are subsequently pushed over by physically rolling over the forest with a cylinder. BP produced in this way has higher electrical and thermal conductivity in the direction of alignment. [20] However, this method is also not amenable to large-scale use, as MWCNT forests with very high degrees of vertical alignment must be grown, a process that is currently only possible in a few laboratories. Ding W, Pengcheng S, Changhong L, Wei W, Shoushan F. Highly oriented carbon nanotube papers made of aligned carbon nanotubes. Nanotechnology. 2008; 19(7):075609. [20] (use of mechanical rolling or “domino pushing” to align CNTs) gives further discussion of this approach, and is incorporated by reference herein.
Magnetic alignment is an alternative method developed by Smalley [21] and refined by Liang and coworkers. [22] This method involves filtering CNTs in the presence of an applied magnetic field. Because CNTs have anisotropic magnetic susceptibilities, they tend to align with the direction of applied magnetic field lines in order to minimize energy. See illustration at FIG. 4. If a sufficiently strong magnetic field is applied to MWCNTs that are very well dispersed in solution, the MWCNTs will become oriented, and subsequent filtering will lead to the formation of aligned BP. Individual nanotubes comprising a MWCNT can be metallic or semiconducting depending on their structure with paramagnetic or diamagnetic responses to applied magnetic fields, respectively, both of which tend to align the MWCNT in the same direction and with nearly the same force. [23-25] However, huge magnetic fields on the order of 10-30 T are required to produce observable degrees of alignment. [21] The cryogenically-cooled electromagnets needed to achieve those massive magnetic fields render this method unfit for the production of aligned BP on any appreciable scale. Additional discussion of the magnetic alignment approach can be found at published patent application US 2002/0185770 to McKague (use of magnetic fields to align CNTs), which is incorporated by reference herein.
McKaque U.S. 2002/0185770 describes some of the issues in this technical field, including the challenges faced trying to achieve CNT alignment. McKaque discusses the potential benefits from such things as an efficient way to produce mass quantities of BP; with an effective degree of CNT alignment to produce anisotropic and other beneficial properties, and the ability to produce composites containing improved loadings of CNTs relative to simple melt or resin mixing.
Thus, the state of the art recognizes there is a need for aligned buckypaper fabrication techniques. But as discussed above, suggested solutions leave room for improvement in terms of flexibility, efficiency, complexity, economy, and applicability to a wide range of types of nanostructures.